Mauna Kea Water Vapour Statistics

Data logged continuously by the 225GHz receiver at the Caltech Submillimeter
Observatory (CSO) over the periods 1994-1995 and 1997-1998 have been analysed. This
equipment measures the zenith optical depth in the wing of an atmospheric water vapour
line and may be used to derive the precipitable H2O column density above the
site (Holland, W. 1999, personal communication):

mm of H2O = 20 * tau(225GHz)

A cumulative frequency histogram of the optical depth was generated for each quarter in
1994 and 1995:

and compared with data for 1997 to mid-1998 data available at CSO.

The following table shows the average optical depths and the values adopted for the
Gemini observing condition constraints:

Period

CSO 225GHz tau

20%-ile

50%-ile

80%-ile

1994-1995

0.047

0.078

0.130

1997Q1

0.048

0.085

0.150

1997Q2

0.052

0.092

0.160

1997Q3

0.068

0.100

0.160

1997Q4

0.041

0.063

0.120

1998Q1

0.037

0.047

0.072

1998Q2

0.042

0.078

0.143

adopted

0.05

0.08

0.15

Thanks are due to Remo Tilanus for extracting the 1994-1995 data from the JCMT archive.